At present, a hearing aid mostly utilizes an induction coil-type (T-coil) sound acquisition device to receive an alternating current (AC) magnetic signal from a receiver of a telephone set. When the telephone is used, a T-coil sensor therein can help a user of the hearing aid to eliminate a background signal audible to human ears, and can also avoid deterioration of the sound quality. When an original-sound type hearing aid and a telephone receiver are used at the same time, the deterioration of the sound quality often occurs. The best solution is triggering a magnetic switch in the hearing aid by using a DC magnetic field in the telephone receiver, to turn off the microphone of the hearing aid and activate the T-coil sound acquisition device, instead of turning off the microphone by using a manual switch.
In addition to improving the sound quality of telephone answering, the hearing aid is gradually applied to an advanced consumer audio system and a public broadcast audio transmission system, a T-coil sensor therein, as a detector of a near-field magnetic communication system, has double functions in a loop system. Generally, an analog audio signal in a near-field magnetic communication system, such as a telephone signal pickup system, a public broadcast system, and a home audio, is carried by a magnetic field, and the magnetic field is very close to a transmission coil. This is different from common wireless transmission in several aspects, among which the biggest difference lies in that: the magnetic field for carrying audio signals in the near-field magnetic communication system is not an electromagnetic wave that is easy to spread. Therefore, the near-field magnetic communication may be conducted only in a room or building, which improves the privacy, thus allowing adjacent systems to respectively transmit information only locally.
A conventional induction coil (T-coil) can only detect an AC magnetic field. There are two types of induction coils, one is a passive type consisting of coils wound on a magnetic core, and the other is an active type including a pre-amplifier. However, an inductor for picking up signals is large in size and expensive. In addition, the induction coil cannot sense the existence of a DC magnetic field, and the existence of the DC magnetic field from a near-field communication device must be detected by using an additional circuit. These devices are big, and occupy a large space in the hearing aid which could be used for other applications of the hearing aid or for increasing the battery space. Another defect of the conventional induction coil-type sound acquisition device is that the conventional induction coil-type sensor is a vector type, instead of a scalar type sensor; therefore, it can only measure magnetic field changes in one direction. Being sensitive to a single axis may be good, but the coil is large in size, and the length of the coil along the sensing axis is greater than the length thereof along the non-sensing axis, such that it is difficult to match the conventional T-Coil with a receiver of a fixed-line telephone.